The invention relates to a magnetic-tape-cassette apparatus, comprising a servo device for positioning at least one apparatus part. The servo device comprises a servo wheel having at least one recessed portion, which faces a motor-driven wheel in a rest position of the servo wheel, and carries an eccentric cam and at least one latching projection. The eccentric cam is subjected to a starting torque produced by spring force in the rest position of the servo wheel. The servo device further comprises a relay-armature member which is pivotable between a latching position and an inoperative position and carries a soft-iron element which in the rest position of the servo wheel is attracted by an impulse relay and thereby maintains the relay-armature member in the latching position. In this position a stop projection on the relay-armature member cooperates with the latching projection on the servo wheel, so that the stop projection keeps the servo wheel latched in the rest position. After a brief electrical energization of the impulse relay, the relay-armature member is pivoted under spring force from the latching position to the inoperative position. In the inoperative position the stop projection is clear of the latching projection and, under the influence of the starting torque exerted on the eccentric cam, the servo wheel is rotated, the circumference of the servo wheel engages the motor-driven wheel, and the servo wheel is driven by the motor to position the apparatus part. The latching projection then cooperates with a reset projection on the relay-armature member, so that the relay-armature member is returned against spring force from the inoperative position to the latching position. Subsequently the latching projection again abuts against the stop projection, and the stop projection again latches the servo wheel in the rest position.
Such a magnetic-tape-cassette apparatus is known from a document published by KISHO-Electronics Co., Ltd. (Japan). The motor-driven wheel shown therein is a gear wheel, and the servo wheel is consequently provided with teeth around its circumference. However, other cooperations between the motor-driven wheel and the servo wheel are alternatively possible, e.g. a cooperation by means of friction, see German Offenlegungsschrift 2,628,287 to which U.S. Pat. No. 4,071,861 corresponds. The magnetic-tape-cassette apparatus proposed therein comprise a relay-armature member which keeps the servo wheel latched in the rest position under the influence of an impulse relay. Such an impulse relay comprises a permanent magnet which in the latching position of the relay-armature member firmly attracts this member by means of the soft-iron element. Around the magnet a coil is arranged to form an electromagnet which can be connected briefly to an electric current source by means of a switch.
The construction and arrangement of the electromagnet relative to the permanent magnet is such that the permanent magnet is rendered magnetically neutral while the current source is switched on. As a result of this, the relay-armature member is no longer attracted and a spring can cause the relay-armature member to be pivoted to the inoperative position, so that the servo wheel is released and can be rotated. The advantage of such an impulse relay is that the current source need only be switched on for a short time, which is adequate to allow the impulse relay to be rendered magnetically neutral for a short time and the relay-armature member to be pivoted. This brief switching-on has the advantage that the current source of the magnetic-tape-cassette apparatus is loaded only briefly and the impulse relay consumes only a small current. This is not only an advantage for battery-powered apparatuses but it is also advantageous for modern microprocessor-controlled equipment, in which such a brief energization is simple to achieve. Moreover, such an impulse relay can be of compact construction and has only a very small heat dissipation in comparison with a conventional electromagnet. The known magnet-tape-cassette apparatus has the drawback that often tape spillage occurs and that the reproduced signal often contains a substantial distortion component.